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Initial sketch of the Paxos alogrithm.

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+.scala_dependencies
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+Implementation of the Paxos algo in Scala based on the following paper.
+
+"Paxos For System Builders"
+Jonathan Kirsch and Yair Amir
+Technical Report CDNS-2008-2 March 2008
+
+My implementation adheres to the paper somewhat closely. The implementation is _not_ of commercial quality. There appears to be a US Patent #5261085 on Paxos by Lamport. My implementation was done for edification purposes, and has been made available under the GLPv3 open source license for the same reasons.
+
+Ray Racine
+July 19, 2009
674 license/LICENSE-GPLV3.txt
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+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+ 16. Limitation of Liability.
+
+ IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+ 17. Interpretation of Sections 15 and 16.
+
+ If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+ END OF TERMS AND CONDITIONS
+
+ How to Apply These Terms to Your New Programs
+
+ If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+ To do so, attach the following notices to the program. It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+ <one line to give the program's name and a brief idea of what it does.>
+ Copyright (C) <year> <name of author>
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+ If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+ <program> Copyright (C) <year> <name of author>
+ This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+ This is free software, and you are welcome to redistribute it
+ under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License. Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+ You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+ The GNU General Public License does not permit incorporating your program
+into proprietary programs. If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library. If this is what you want to do, use the GNU Lesser General
+Public License instead of this License. But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.
8 project/build.properties
@@ -0,0 +1,8 @@
+#Project properties
+#Sat May 23 08:59:20 GMT-05:00 2009
+project.organization=empty
+project.name=Paxos
+sbt.version=0.5.1
+project.version=1.0
+scala.version=2.7.5
+project.initialize=false
193 src/main/scala/com/rracine/paxos/Communication.scala
@@ -0,0 +1,193 @@
+/*
+ * Copyright 2009-2010 Ray Racine
+
+ * This file is part of Crank.
+
+ * Crank is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the LicenSe, or
+ * (at your option) any later version.
+
+ * Crank is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with Crank. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.rracine.paxos
+
+import scala.actors.Actor
+
+trait Sender {
+
+ def sendToAll (msg: Message): Unit
+
+ def sendToLeader (msg: Message): Unit
+
+ def replyToClient (): Unit
+
+ def register (actor: Actor): Unit
+
+}
+
+object MC {
+ val DEF_PORT = 6063
+ val DEF_GROUP = "228.5.6.7"
+}
+
+object MsgId {
+ val VC_ID: Byte = 1
+ val VCP_ID: Byte = 2
+ val PREP_ID: Byte = 3
+ val PREPOK_ID: Byte = 4
+ val ACC_ID: Byte = 5
+ val PROP_ID: Byte = 6
+ val GO_ID: Byte = 7
+ val CU_ID: Byte = 8
+}
+
+object MsgBuffer {
+
+ import MsgId._
+ import Paxos.{ARU, ServerId, View, Sequence, Command, TimeStamp, DataList}
+
+ import java.nio.ByteBuffer
+
+ type Buffer = ByteBuffer
+
+ def allocate (sz: Int): Buffer =
+ ByteBuffer.allocate (sz)
+
+ def deserialize (buffer: Array[Byte]): Message = {
+ val bb = ByteBuffer.wrap (buffer)
+
+ bb.get match {
+ case `ACC_ID` => Accept (bb.getInt, bb.getInt, bb.getInt)
+ case `CU_ID` => ClientUpdate (bb.getInt, bb.getInt, bb.getLong, "FIXME - NO COMMAND")
+ case `VC_ID` => ViewChange (bb.getInt, bb.getInt)
+ case `VCP_ID` => ViewChangeProof (bb.getInt, bb.getInt)
+ case `PREP_ID` => Prepare (bb.getInt, bb.getInt, bb.getInt)
+ case `PREPOK_ID` => PrepareOK (bb.getInt, bb.getInt, bb.getInt, Nil)
+ case `PROP_ID` => Proposal (bb.getInt, bb.getInt, bb.getInt, "NO CMD")
+ case b => throw new Exception ("Unknown message id: " + b)
+ }
+ }
+
+ def serializeCmd (cmd: Paxos.Command, buffer: Buffer): Buffer = {
+ cmd.foreach (buffer.putChar (_))
+ buffer
+ }
+
+ def serialize (msg: ClientUpdate, buffer: Buffer): Buffer = {
+ buffer.put (CU_ID).putInt (msg.clientId).putInt (msg.serverId).putLong (msg.ts)
+ serializeCmd (msg.cmd, buffer)
+ }
+
+ def serialize (msg: ViewChange, buffer: Buffer): Buffer =
+ buffer.put (VC_ID).putInt (msg.serverId).putInt (msg.attempted)
+
+ def serialize (msg: ViewChangeProof, buffer: Buffer): Buffer =
+ buffer.put (VCP_ID).putInt (msg.serverId).putInt (msg.installed)
+
+ def serialize (msg: Prepare, buffer: Buffer): Buffer =
+ buffer.put (PREP_ID).putInt (msg.serverId).putInt (msg.view).putInt (msg.aru)
+
+ def serialize (msg: PrepareOK, buffer: Buffer): Buffer =
+ buffer.put (PREPOK_ID).putInt (msg.serverId).putInt (msg.leaderId).putInt (msg.view) // FIXME RPR data
+
+ def serialize (msg: Accept, buffer: Buffer): Buffer =
+ buffer.put (ACC_ID).putInt (msg.serverId).putInt (msg.view).putInt (msg.seq)
+
+ def serialize (msg: Proposal, buffer: Buffer): Buffer = {
+ buffer.put (PROP_ID).putInt (msg.serverId).putInt (msg.view).putInt (msg.seq)
+ serializeCmd (msg.cmd, buffer)
+ }
+
+ def serialize (msg: GloballyOrderedUpdate, buffer: Buffer): Buffer = {
+ buffer.put (GO_ID).putInt (msg.serverId).putInt (msg.seq)
+ serializeCmd (msg.cmd, buffer)
+ }
+}
+
+/**
+ * Sends and Receives Messages.
+ * Messages are sent via normal method calls on the invoker's thread.
+ * However inbound messages are on an Actor's thread and are placed
+ * on the PaxosServer's Actor message queue for processing.
+ */
+class UDPMulticastCommunication () extends AnyRef with Sender {
+
+ import MC.{DEF_PORT, DEF_GROUP}
+
+ import java.net.{InetAddress, MulticastSocket, DatagramPacket}
+
+ val BUFF_SZ = 128
+
+ private val buffer = MsgBuffer.allocate (BUFF_SZ)
+
+ val group = InetAddress.getByName (MC.DEF_GROUP)
+
+ private var paxos: Actor = null
+
+ private val socket = {
+ val s = new MulticastSocket (MC.DEF_PORT)
+ s.joinGroup (group)
+ s
+ }
+
+ private val receiverThread = new Thread (new Runnable () {
+ override def run (): Unit = {
+ //val s = new MulticastSocket (MC.DEF_PORT)
+ val buff = MsgBuffer.allocate (BUFF_SZ)
+ //println ("Joining broadcast group")
+ //s.joinGroup (group)
+ while (true) {
+ buff.clear
+ val packet = new DatagramPacket (buff.array, buff.limit)
+ socket.receive (packet)
+ try {
+ val msg = MsgBuffer.deserialize (packet.getData)
+ paxos ! msg
+ } catch {
+ case e: Exception => println ("Deserialize failed: " + e.getMessage)
+ }
+ }
+ }
+ }, "Multicast Receiver")
+
+ receiverThread.setDaemon (true)
+ receiverThread.start
+
+ def serializeMsg (msg: Message, buffer: MsgBuffer.Buffer): MsgBuffer.Buffer =
+ msg match {
+ case m: ViewChange => MsgBuffer.serialize (m, buffer)
+ case m: ViewChangeProof => MsgBuffer.serialize (m, buffer)
+ case m: Prepare => MsgBuffer.serialize (m, buffer)
+ case m: PrepareOK => MsgBuffer.serialize (m, buffer)
+ case m: Proposal => MsgBuffer.serialize (m, buffer)
+ case m: ClientUpdate => MsgBuffer.serialize (m, buffer)
+ case m: Accept => MsgBuffer.serialize (m, buffer)
+ case m: GloballyOrderedUpdate => MsgBuffer.serialize (m, buffer)
+ }
+
+ def sendToAll (msg: Message): Unit = {
+ buffer.clear
+ val buff = serializeMsg (msg, buffer)
+ socket.send (new DatagramPacket(buff.array, buff.position, group, DEF_PORT))
+ }
+
+ def sendToLeader (msg: Message): Unit =
+ sendToAll (msg)
+
+ def replyToClient (): Unit =
+ println ("Reply to client not implemented.")
+
+ def register (actor: Actor) =
+ paxos = actor
+}
+
+
+
101 src/main/scala/com/rracine/paxos/GlobalHistory.scala
@@ -0,0 +1,101 @@
+/*
+ * Copyright 2009-2010 Ray Racine
+
+ * This file is part of Crank.
+
+ * Crank is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the LicenSe, or
+ * (at your option) any later version.
+
+ * Crank is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with Crank. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.rracine.paxos
+
+import scala.collection.mutable.HashMap
+
+import Paxos.Sequence
+
+sealed abstract class GlobalHistoryEntry
+case class GOProposal (proposal: Proposal, accepts : HashMap[Paxos.ServerId, Accept]) extends GlobalHistoryEntry
+case class GOConsensus (entry: GloballyOrderedUpdate) extends GlobalHistoryEntry
+
+/**
+ * Window sliding history of entries.
+ * Constant time access.
+ */
+
+class GlobalHistory {
+
+ /** Current size of the window */
+ val windowSize: Int = 1024 // base 2 for efficient modulo operation
+
+ /** The history window. */
+ val window: Array[GlobalHistoryEntry] = new Array[GlobalHistoryEntry] (windowSize)
+
+ /** Window base */
+ var base = 0
+
+ def isEmpty (seq: Sequence): Boolean =
+ this (seq) == null
+
+ def isOrdered (seq: Sequence): Boolean =
+ historyEntry (seq) match {
+ case Some (e: GOConsensus) => true
+ case Some (e: GOProposal) => false
+ case None => false
+ }
+
+ def historyEntry (seq: Sequence): Option[GlobalHistoryEntry] = {
+ val idx = seq - base
+ if (idx < windowSize) {
+ val entry = window (idx)
+ if (entry == null) None else Some (window (idx))
+ }
+ else None
+ }
+
+ def hasEntry (seq: Sequence): Boolean =
+ historyEntry (seq).isDefined
+
+ def proposal (seq: Sequence): Option[GOProposal] =
+ historyEntry (seq) match {
+ case Some (gop: GOProposal) => Some (gop)
+ case _ => None
+ }
+
+ def hasProposal (seq: Sequence): Boolean =
+ historyEntry (seq) match {
+ case Some (_: GOProposal) => true
+ case _ => false
+ }
+
+ /** Assure n slots are available. */
+ def assure (n: Int): Unit = {
+ var idx = n
+ while (n > 0) {
+ assert (isOrdered (base))
+ base += 1
+ }
+ }
+
+ /** Access ith element of history */
+ def apply (idx: Int): GlobalHistoryEntry = {
+ assert (idx - base < windowSize)
+ val entry = window (idx % windowSize)
+ assert (entry != null)
+ entry
+ }
+
+ def update (idx: Int, entry: GlobalHistoryEntry) = {
+ assert (idx - base < windowSize)
+ window (idx % windowSize) = entry
+ }
+}
97 src/main/scala/com/rracine/paxos/Main.scala
@@ -0,0 +1,97 @@
+/*
+ * Copyright 2009-2010 Ray Racine
+
+ * This file is part of Crank.
+
+ * Crank is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the LicenSe, or
+ * (at your option) any later version.
+
+ * Crank is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with Crank. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.rracine.paxos
+
+import java.lang.System.{currentTimeMillis => now}
+
+class TestCluster () extends AnyRef with Cluster {
+
+ def replyToClient (): Unit =
+ println ("Reply to client - unfinished.")
+
+ def nodeCount (): Int = 3
+
+}
+
+class PaxosServer (id: Int) extends Server (id, new TestCluster (), new UDPMulticastCommunication ()) {
+
+ import Paxos.{ARU, ServerId, View, Sequence, Command, TimeStamp, DataList}
+
+ def globalHistoryHasProposal (seq: Sequence): Boolean = {
+ println ("Global history has proposal - unfinished.")
+ false
+ }
+
+ def pendingUpdatesContains (cmd: Command): Boolean = {
+ println ("Pending updates contains - unfinished.")
+ false
+ }
+
+ def setUpdateTimer (id: ServerId): Unit = ()
+ //println ("Set update timer - unfinished.")
+
+ def cancelUpdateTimer (id: ServerId): Unit = ()
+
+ def isBound (msg: ClientUpdate): Boolean = true
+
+ def clearUpdateQueue (): Unit =
+ println ("Clear update queue - unfinished.")
+
+ def pendingUpdatesRemove (cmd: Command): Boolean = {
+ println ("Pending updates removed - unfinished.")
+ true
+ }
+
+}
+
+object Main {
+
+ def sendClientMsgs (n: Int, server: Server): Unit = {
+ var i = 0
+ while (true) {
+ Thread.sleep (1000,0)
+ if (server.state != LEADER_ELECTION && server.myServerId == server.leaderId) {
+ val s = "Client Msg: " + i
+ println (s)
+ i = i + 1
+ val msg = ClientUpdate (server.myServerId * 100 + i, server.myServerId, now, s)
+ server.clientUpdateHandler (msg)
+ }
+ }
+ }
+
+ def parseServerId (args: Array[String]) = args (0).toInt
+
+ def main (args: Array[String]): Unit = {
+ try {
+ val server = new PaxosServer (parseServerId (args))
+ println ("Starting Paxos Server - " + server.myServerId)
+ server.start
+ Thread.sleep (1000, 0) // yield (can't yield!!)
+ server ! InitServer
+ sendClientMsgs (100, server)
+ } catch {
+ case e => {
+ e.printStackTrace
+ }
+ }
+ }
+
+}
546 src/main/scala/com/rracine/paxos/Paxos.scala
@@ -0,0 +1,546 @@
+/*
+ * Copyright 2009-2010 Ray Racine
+
+ * This file is part of Crank.
+
+ * Crank is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the LicenSe, or
+ * (at your option) any later version.
+
+ * Crank is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with Crank. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.rracine.paxos
+
+import java.lang.System.{currentTimeMillis => now}
+import scala.collection.mutable.{ArrayBuffer, HashMap, Queue}
+
+trait Cluster {
+
+ /** Number of nodes in the cluster */
+ def nodeCount (): Int
+
+ /** Count of nodes for a quorum. */
+ def quorum (): Int = peerQuorum + 1
+
+ /**
+ * Number of peers necessary to make a quorum.
+ * This is because this node counts itself.
+ */
+ def peerQuorum (): Int = nodeCount / 2
+
+ def replyToClient (): Unit
+}
+
+object Paxos {
+
+ // Unique server identifier.
+ type ServerId = Int
+
+ // The current view identifier.
+ type View = Int
+
+ type Sequence = Int
+
+ // The synchronized command
+ type Command = String
+
+ type TimeStamp = Long
+ val MIN_TS: TimeStamp = 0
+ val MAX_TS: TimeStamp = Math.MAX_LONG
+
+ // List of Proposals and GlobalOrderUpdates
+ type DataList = List[Message]
+
+ //
+ type ARU = Int
+}
+
+import Paxos.{ARU, ServerId, View, Sequence, Command, TimeStamp, DataList}
+
+
+sealed case class Message ()
+
+case object InitServer
+case object ProgressTimerExpiration
+
+case class ClientUpdate (clientId: ServerId, serverId: ServerId, ts: TimeStamp, cmd: Command) extends Message
+case class ViewChange (serverId: ServerId, attempted: View) extends Message
+case class ViewChangeProof (serverId: ServerId, installed: View) extends Message
+case class Prepare (serverId: ServerId, view: View, aru: ARU) extends Message
+case class PrepareOK (serverId: ServerId, leaderId: ServerId, view: View, data: Paxos.DataList) extends Message
+case class Proposal (serverId: ServerId, view: View, seq: Sequence, cmd: Command) extends Message
+case class Accept (serverId: ServerId, view: View, seq: Sequence) extends Message
+case class GloballyOrderedUpdate (serverId: ServerId, seq: Sequence, cmd: Command) extends Message
+
+sealed case class State ()
+case object LEADER_ELECTION extends State
+case object REG_LEADER extends State
+case object REG_NONLEADER extends State
+
+import scala.actors.Actor
+import scala.actors.Actor.loop
+
+abstract class Server (val myServerId: ServerId, cluster: Cluster, send: Sender) extends AnyRef with Actor {
+
+ /* View related state. */
+ var lastAttempted: View = _
+ var lastInstalled: View = _
+ val viewChanges: HashMap[ServerId, ViewChange] = HashMap.empty
+
+ var state: State = LEADER_ELECTION
+ val prepareOKs: Array[Option[PrepareOK]] = Array.make[Option[PrepareOK]] (cluster.nodeCount, None)
+
+ val globalHistory = new GlobalHistory ()
+
+ val lastEnqueued: HashMap[ServerId, TimeStamp] = HashMap.empty
+
+ // Global ordering variables
+ var lastProposed: Sequence = _
+ var localARU: ARU = _
+
+ val lastExecuted: HashMap[ServerId, TimeStamp] = HashMap.empty
+
+ val updateQueue: Queue[ClientUpdate] = new Queue ()
+
+ /** Client update messages indexed by the client id. */
+ val pendingUpdates: HashMap[ServerId, ArrayBuffer[ClientUpdate]] = HashMap.empty
+
+ def leaderId (): ServerId =
+ lastInstalled % cluster.nodeCount
+
+ def check (msg: Message, reason: String, bool: Boolean): Unit =
+ if (bool) println (" Message conflict for " + msg + ". Reason: " + reason)
+
+ def conflictViewChange (msg: ViewChange): Boolean = {
+ check (msg, "Self message", msg.serverId == myServerId)
+ check (msg, "Not in LeaderElection", state != LEADER_ELECTION)
+ check (msg, "PT is set", PaxosTimer.isSetProgressTimer)
+ check (msg, "Attempt is less then Last Installed.", msg.attempted <= lastInstalled)
+ msg.serverId == myServerId || state != LEADER_ELECTION || PaxosTimer.isSetProgressTimer || msg.attempted <= lastInstalled
+ }
+
+ /**
+ * Check incoming messages. All conflicted messages are discarded.
+ */
+ def messageConflict (msg: Message): Boolean =
+ msg match {
+ // Below from paper. Problem is for a Cluster of 1 our VC msg is never processed.
+ case msg @ ViewChange (serverId, attempted) => conflictViewChange (msg)
+ case ViewChangeProof (serverId, _) => serverId == myServerId || state != LEADER_ELECTION
+ case Prepare (serverId, view, aru) => serverId == myServerId || view != lastAttempted
+ case PrepareOK (serverId, leaderId, view, _) => leaderId != myServerId || serverId == myServerId || state != LEADER_ELECTION || view != lastAttempted
+ case Proposal (serverId, view, _, update) => serverId == myServerId || state != REG_NONLEADER || view != lastInstalled
+ case Accept (serverId, view, seq) => serverId == myServerId || view != lastInstalled || globalHistory.hasProposal (seq)
+ case _ => false
+ }
+
+ /**
+ * Track number of peers who agree on the new view.
+ * Returns whether the VC was applied or not
+ * since multiple VCs can be received because of
+ * rebroadcast for msg loss and timing considerations
+ */
+ def updateViewChange (msg: ViewChange): Boolean =
+ if (viewChanges contains msg.serverId)
+ false
+ else {
+ viewChanges update (msg.serverId, msg)
+ true
+ }
+
+ /**
+ * Generic Global Order History Data Structure Mutation Method
+ * The following mutate global state based upon the message being processed.
+ * Pattern match and dispatch to allow recursive processing of messages.
+ */
+ def updateDataStructures (msg: Message): Unit =
+ msg match {
+
+ case msg @ PrepareOK (serverId, _, _, data) =>
+ if (prepareOKs (serverId).isEmpty) {
+ prepareOKs (serverId) = Some (msg)
+ for (e <- data) {
+ updateDataStructures (e)
+ }
+ }
+
+ case pnew @ Proposal (_, view, seq, _) =>
+ globalHistory.proposal (seq).foreach ((gop) =>
+ if (view > gop.proposal.view)
+ globalHistory (seq) = GOProposal (pnew, HashMap.empty))
+
+ case msg @ Accept (serverId, _, seq) =>
+ globalHistory.proposal (seq).foreach ((gop) =>
+ if ((gop.accepts.size < cluster.peerQuorum) && (!gop.accepts.contains (serverId))) {
+ gop.accepts (serverId) = msg
+ })
+
+ case msg @ GloballyOrderedUpdate (_, seq, _) =>
+ if (!globalHistory.isOrdered (seq))
+ globalHistory.update (seq, GOConsensus (msg))
+
+ case msg => println ("ERROR: Misapplying a message to update global data structures: " + msg)
+ }
+
+ /**
+ * Process a received ViewChange message.
+ * If the ViewChange is higher then our last attempt
+ * move to the higher View and enter LeaderElection.
+ *
+ * If the view is equal to what the server is attempting
+ * then count that server as part of the quorum for the
+ * new view.
+ *
+ * If a quorum of servers have agreed on the new view
+ * shift to the prepare phase.
+ */
+ def processViewChange (vc: ViewChange): Unit = {
+ if (vc.attempted > lastAttempted && !PaxosTimer.isSetProgressTimer) {
+ shiftToLeaderElection (vc.attempted)
+ updateViewChange (vc)
+ } else if (vc.attempted == lastAttempted && updateViewChange (vc) && viewPreinstallReady (vc.attempted)) {
+ PaxosTimer ! PTReset2x
+ if (leaderOfView (lastAttempted))
+ shiftToPreparePhase
+ }
+ }
+
+ /**
+ * A ViewChangeProof means a majority outside of ourselves
+ * have agreed on a new view already.
+ * Go ahead and join the cluster under that view.
+ */
+ def processViewChangeProof (msg: ViewChangeProof): Unit =
+ if (msg.installed > lastInstalled) {
+ lastAttempted = msg.installed
+ if (leaderOfView (lastAttempted))
+ shiftToPreparePhase
+ else shiftToRegNonLeader
+ }
+
+ /**
+ * Process a received Prepare message.
+ * If we are still electing give it up.
+ * A prepare means a majority elected a leader so go ahead and move to out of election
+ * and respond that we are ready for the new leader.
+ */
+ def processPrepare (msg: Prepare): Unit =
+ if (state == LEADER_ELECTION) {
+ val datalist = constructDataList (msg.aru)
+ val okMsg = PrepareOK (myServerId, msg.serverId, msg.view, datalist)
+ prepareOKs (myServerId) = Some (okMsg)
+ shiftToRegNonLeader
+ send.sendToLeader (okMsg)
+ } else prepareOKs (myServerId).map ((msg) => send.sendToLeader (msg))
+
+ /**
+ * Process a received PrepareOK message.
+ * If a majority agree become the leader.
+ *
+ * If we fail to get elected leader our
+ * ProgressTimer will expire and back to
+ * leader election we go.
+ */
+ def processPrepareOK (msg: PrepareOK): Unit = {
+ updateDataStructures (msg)
+ if (viewPreparedReady (msg.view))
+ shiftToRegLeader
+ }
+
+ def processAccept (msg: Accept): Unit = {
+ updateDataStructures (msg)
+ globallyOrderedReady (msg.seq).foreach ((gop) => {
+ updateDataStructures (GloballyOrderedUpdate (gop.proposal.serverId,
+ msg.seq,
+ gop.proposal.cmd))
+ })
+ advanceARU
+ }
+
+ def processProposal (msg: Proposal) = {
+ updateDataStructures (msg)
+ val accept = Accept (myServerId, msg.view, msg.seq)
+ syncToDisk
+ send.sendToAll (accept)
+ }
+
+ def advanceARU (): Unit = {
+ var idx = localARU + 1
+ while (true) {
+ if (globalHistory.isOrdered (idx)) {
+ localARU += 1
+ idx += 1
+ } else return
+ }
+ }
+
+ def sendProposal (): Unit = {
+ val seq = lastProposed + 1
+
+ if (globalHistory.isOrdered (seq)) {
+ lastProposed += 1
+ sendProposal // Scala is tail recursive here.
+ } else { // Different from paper added after email with J.K.
+ val cmd = globalHistory.proposal (seq) match {
+ case Some (gop: GOProposal) => gop.proposal.cmd
+ case None =>
+ if (updateQueue.isEmpty)
+ return
+ else updateQueue.dequeue.cmd
+ }
+
+ // Paper has 'view', should be lastInstalled. Confirmed with J.K.
+ val proposal = Proposal (myServerId, lastInstalled, seq, cmd)
+ updateDataStructures (proposal)
+ lastProposed = seq
+ syncToDisk
+ send.sendToAll (proposal)
+ }
+ }
+
+ /**
+ * A proposal is ready for global ordering if a majority of the
+ * cluster has accepted the proposal.
+ */
+ def globallyOrderedReady (seq: Sequence): Option[GOProposal] =
+ globalHistory.proposal (seq).filter (_.accepts.size >= cluster.peerQuorum)
+
+ def postProcessClientUpdate (msg: ClientUpdate): Unit = {
+ advanceARU
+ if (msg.serverId == myServerId) {
+ cluster.replyToClient
+ if (pendingUpdates.contains (msg.clientId)) {
+ cancelUpdateTimer (msg.clientId)
+ pendingUpdates - (msg.clientId)
+ }
+ }
+ lastExecuted (msg.clientId) = now
+ if (state != LEADER_ELECTION)
+ PaxosTimer ! PTReset
+ if (state == REG_LEADER)
+ sendProposal
+ }
+
+ def leaderOfView (view: View): Boolean =
+ myServerId == view % cluster.nodeCount
+
+ /** Callback method invoked on progress timer expiration. */
+ def progressTimerExpiration (): Unit = {
+ println ("Progress Timer expiration: shifting to leader election.")
+ shiftToLeaderElection (lastAttempted + 1)
+ }
+
+ /* Start ViewChangeProof periodic timer */
+ def startVCProofTimer (): Unit =
+ PaxosTimer ! SendViewChangeProof
+
+ /**
+ * Progress has not been made.
+ * Prepare and elect a new leader.
+ */
+ def shiftToLeaderElection (view: View): Unit = {
+ state = LEADER_ELECTION
+ viewChanges.clear; lastEnqueued.clear; lastAttempted = view
+
+ // clear prepareOKs
+ for (i <- 0 until prepareOKs.size) {
+ prepareOKs (i) = None
+ }
+
+ val vc = ViewChange (myServerId, lastAttempted)
+ send.sendToAll (vc)
+ updateViewChange (vc)
+ println ("Starting Periodic VC")
+ PaxosTimer ! SendViewChange
+ }
+
+ def cancelUpdateTimer (clientId: ServerId): Unit
+
+ def syncToDisk (): Unit = ()
+
+ def pendingUpdatesContains (cmd: Command): Boolean
+
+ def pendingUpdatesRemove (cmd: Command): Boolean
+
+ def shiftToRegLeader (): Unit = {
+ state = REG_LEADER
+ PaxosTimer ! StopViewChange
+ PaxosTimer ! PTReset
+ enqueueUnboundPendingUpdates
+ removeBoundUpdatesFromQueue
+ lastProposed = localARU
+ sendProposal
+ }
+
+ def clearUpdateQueue (): Unit
+
+ def shiftToRegNonLeader (): Unit = {
+ state = REG_NONLEADER
+ lastInstalled = lastAttempted
+ PaxosTimer ! StopViewChange
+ PaxosTimer ! PTReset
+ clearUpdateQueue
+ syncToDisk
+ }
+
+ def clearLastEnqueued (): Unit =
+ lastEnqueued.clear
+
+ /** Fix Me - Figure 8 */
+ def constructDataList (aru: ARU) = Nil
+
+ var preparedCount = 1
+
+ def viewPreparedReady (view: View): Boolean = {
+ val quorum = cluster.quorum
+ var preparedCount = 0
+
+ for (pokOpt <- prepareOKs) {
+ pokOpt.map ((pok) => if (pok.view == view) preparedCount += 1)
+ if (preparedCount == quorum) return true
+ }
+ return false
+ }
+
+ /**
+ * Ready to pre-install a view of a majority of the cluster agree on a new view.
+ */
+ def viewPreinstallReady (view: View): Boolean =
+ viewChanges.size >= cluster.quorum
+
+ def shiftToPreparePhase (): Unit = {
+ lastInstalled = lastAttempted
+ val prepare = Prepare (myServerId, lastInstalled, localARU)
+ val datalist = constructDataList (localARU)
+ val prepareOK = PrepareOK (myServerId, myServerId, lastInstalled, datalist)
+ prepareOKs (myServerId) = Some (prepareOK)
+ clearLastEnqueued
+ syncToDisk
+ send.sendToAll (prepare)
+ }
+
+ /*=======================
+ * Client Update Handling
+ *======================= */
+ def clientUpdateHandler (msg: ClientUpdate): Unit = {
+ state match {
+ case LEADER_ELECTION =>
+ if (msg.serverId != myServerId && enqueueUpdate (msg)) {
+ println ("CU - LE")
+ //addToPendingUpdates (msg) All these temp removed
+ }
+
+ case REG_NONLEADER =>
+ if (msg.serverId == myServerId) {
+ //addToPendingUpdates (msg)
+ println ("CU - RN")
+ send.sendToLeader (msg)
+ ()
+ }
+
+ case REG_LEADER =>
+ if (enqueueUpdate (msg)) {
+ //if (msg.serverId == myServerId)
+ // addToPendingUpdates (msg)
+ println ("CU - RL")
+ sendProposal
+ }
+ }
+ }
+
+ // FIXME
+ def isBound (update: ClientUpdate): Boolean
+
+ def addToPendingUpdates (msg: ClientUpdate): Unit = {
+ // FIXME FOR LOOKUP FAILS
+ //pendingUpdates (msg.clientId) += msg
+ setUpdateTimer (msg.clientId)
+ syncToDisk
+ }
+
+ def setUpdateTimer (clientId: ServerId): Unit
+
+ def enqueueUpdate (msg: ClientUpdate): Boolean =
+ if (msg.ts <= lastExecuted.getOrElse (msg.clientId, Paxos.MIN_TS) ||
+ msg.ts <= lastEnqueued.getOrElse (msg.clientId, Paxos.MIN_TS))
+ false
+ else {
+ updateQueue += msg
+ lastEnqueued.update (msg.clientId, msg.ts)
+ true
+ }
+
+ def enqueueUnboundPendingUpdates (): Unit =
+ for ((_,buffer) <- pendingUpdates) {
+ for (u <- buffer) {
+ if (!isBound (u) || !updateQueue.contains (u))
+ enqueueUpdate (u)
+ }
+ }
+
+ def removeBoundUpdatesFromQueue (): Unit =
+ updateQueue.dequeueAll ((u: ClientUpdate) =>
+ if (isBound (u) || (u.ts <= lastExecuted.getOrElse (u.clientId, Paxos.MIN_TS)) ||
+ (u.ts <= lastEnqueued.getOrElse (u.clientId, Paxos.MIN_TS) && u.serverId != myServerId)) {
+ if (u.ts > lastEnqueued.getOrElse (u.clientId, Paxos.MIN_TS))
+ lastEnqueued (u.clientId) = u.ts
+ true
+ } else false)
+
+ /**
+ * The heart of the Paxos Server. Process msgs off msg queue.
+ * An aspect of processing a message is mutation of state.
+ * All state mutating message processing is encapsulated by
+ * the updateDataStructures driver method.
+ */
+ def act (): Unit = {
+ loop {
+ println ("State: " + state + " lastInstalled: " + lastInstalled + " lastAttempted: " + lastAttempted)
+ react {
+ case SendViewChangeProof =>
+ send.sendToAll (ViewChangeProof (myServerId, lastInstalled))
+
+ case SendViewChange => {
+ val msg = ViewChange (myServerId, lastAttempted)
+ println ("Sending periodic ViewChange: " + msg)
+ send.sendToAll (msg)
+ }
+
+ case msg: Message => { // nested for msg conflict handling
+ println ("Msg: " + msg)
+ if (!messageConflict (msg))
+ msg match {
+ case msg: Accept => processAccept (msg)
+ case msg: ViewChange => processViewChange (msg)
+ case msg: ViewChangeProof => processViewChangeProof (msg)
+ case msg: Prepare => processPrepare (msg)
+ case msg: PrepareOK => processPrepareOK (msg)
+ case msg: ClientUpdate => clientUpdateHandler (msg)
+ case msg: Proposal => processProposal (msg)
+ case msg @ _ => println ("Unhandled Message: " + msg.toString)
+ } else println (" -- Dropped --")
+ }
+
+ case ProgressTimerExpiration => {
+ progressTimerExpiration
+ }
+
+ case InitServer => {
+ progressTimerExpiration
+ startVCProofTimer
+ }
+ case msg @ _ => println ("Unknown msg: " + msg)
+ }
+ }
+ }
+
+ send.register (this)
+}
168 src/main/scala/com/rracine/paxos/PaxosTimer.scala
@@ -0,0 +1,168 @@
+/*
+ * Copyright 2009-2010 Ray Racine
+
+ * This file is part of Crank.
+
+ * Crank is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the LicenSe, or
+ * (at your option) any later version.
+
+ * Crank is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+
+ * You should have received a copy of the GNU General Public License
+ * along with Crank. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.rracine.paxos
+
+import scala.actors.Actor
+import scala.actors.Actor.loop
+
+sealed case class TimerMessage ()
+
+import java.util.TimerTask
+
+// Reset ProgressTimer
+case object PTReset extends TimerMessage
+// Reset ProgressTimer to 2x its remaining value.
+case object PTReset2x extends TimerMessage
+case class PTStop (tt: TimerTask) extends TimerMessage
+
+/**
+ * Sent by the Paxos Server to the PaxosTImer to starts periodic timer.
+ * Sent by the PaxosTimer to the PaxosServer to cause a ViewChangeProof sendToAll.
+ */
+case object SendViewChangeProof extends TimerMessage
+
+case object SendViewChange extends TimerMessage
+case object StopViewChange extends TimerMessage
+
+/**
+ * Progress Timer to ensure the cluster is making progress.
+ * Implemented as an Actor.
+ *
+ * The progressTimerTask was marked as volatile, however, the Paxos Server is a single
+ * threaded beast and the only thing which invokes ProgressTimer methods.
+ * i.e., ProgressTimer never sees more than one thread externally.
+ *
+ * The Timer thread affects both ProgressTimer and the PaxosServer state.
+ * However, the Timer thread interacts with both via Actor messages.
+ * Hopefully, leaving us safe from concurrency and parallism harm.
+ *
+ * The Paxos Server is an Actor processing messages on a single thread.
+ * PaxosTimer runs a scheduled periodic VCProof timer task.
+ * Upon task expiration the SendViewChangeProof message
+ * is dropped on the Paxos Server message queue.
+ */
+
+object PaxosTimer extends AnyRef with Actor {
+
+ import java.util.Timer
+ import java.util.concurrent.atomic.AtomicInteger
+ import java.lang.System.{currentTimeMillis => now}
+
+ private val PT_DEFAULT_TIMEOUT = 15000 // millis
+ private val VC_PROOF_PERIOD = 2000
+
+ private var ptTimeout = PT_DEFAULT_TIMEOUT
+ private val timer: Timer = new Timer ("PaxosTimer", false)
+
+ /**
+ * The task to be executed upon expiration.
+ * The Option-al aspect is used to determine if the ProgressTimer is set.
+ * Some - ProgressTimer is set.
+ * None - ProgressTimer is not set.
+ */
+ private var progressTimerTask: Option[TimerTask] = None
+
+ /**
+ * Sends periodic ViewChange events back to Server until cancelled.
+ */
+ private var viewchangeTimerTask: Option[TimerTask] = None
+
+ private def progressTimerStop (): Unit =
+ progressTimerTask.foreach ((tt) => {
+ progressTimerTask = None
+ tt.cancel
+ timer.purge
+ })
+
+ private def progressTimerReset (invoker: Actor, timeout: Long) = {
+ progressTimerStop
+ progressTimerTask = Some (new TimerTask {
+ def run (): Unit = {
+ PaxosTimer ! PTStop (this)
+ invoker ! ProgressTimerExpiration
+ }
+ })
+ timer.schedule (progressTimerTask.get, timeout)
+ }
+
+ private def stopViewChange () =
+ viewchangeTimerTask.foreach ((tt) => {
+ viewchangeTimerTask = None
+ tt.cancel
+ timer.purge
+ })
+
+ def act (): Unit =
+ loop {
+ react {
+
+ case PTStop (ttexp) =>
+ progressTimerTask.foreach ((tt) => if (ttexp == tt) progressTimerStop)
+
+ case PTReset => {
+ ptTimeout = PT_DEFAULT_TIMEOUT
+ progressTimerReset (sender.receiver, PT_DEFAULT_TIMEOUT)
+ }
+ case PTReset2x => {
+ ptTimeout = ptTimeout * 2
+ progressTimerReset (sender.receiver, ptTimeout)
+ }
+
+ case SendViewChange => {
+ stopViewChange
+ val invoker = sender.receiver
+ viewchangeTimerTask = Some (new TimerTask {
+ def run (): Unit = {
+ invoker ! SendViewChange
+ }
+ })
+ timer.schedule (viewchangeTimerTask.get, VC_PROOF_PERIOD, VC_PROOF_PERIOD)
+ }
+
+ case StopViewChange => stopViewChange
+
+ case SendViewChangeProof => {
+ val invoker = sender.receiver
+ timer.schedule (new TimerTask {
+ def run (): Unit = {
+ invoker ! SendViewChangeProof
+ }
+ }, VC_PROOF_PERIOD, VC_PROOF_PERIOD)
+ }
+
+ }
+ }
+
+ /**
+ * Whether the ProgressTime has or has not been set.
+ * This is the only method that is not invoked via an Actor message.
+ * Therefore it is the only place where 2 threads have shared state.
+ * This state is only mutated on the Actor thread and is read-only
+ * to the invoker of isSet.
+ *
+ * Since the PaxosServer is a single thread beast and the ProgressTimer Actor
+ * acts on the invoker thread (reacts and not receives) there is really only
+ * one thread involved here. The timerTask var doesn't?? need to be volatile.
+ */
+ def isSetProgressTimer (): Boolean =
+ progressTimerTask.isDefined
+
+ start ()
+}
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